Presently RF and microwave signals are mainly measured by high frequency (HF) spectrum analyzers, real-time and repetitive sampling HF oscilloscopes and HF vector signal analyzers. HF spectrum analyzers provide the amplitude spectrum of a signal as function of the frequency. HF real-time oscilloscopes use very broadband and fast-sampling data acquisition systems, usually limited by the number of bits. They also require deep memory to measure a high frequency tone, e.g. 5 GHz, which is slowly modulated, e.g. a modulation signal with a bandwidth of kHz or MHz compared to the 5 GHz of the carrier. Repetitive sampling oscilloscopes use a very broadband sampler, functioning as a sample and hold circuit, and a slower data acquisition system. They suffer from the dynamic range due to the downfolding of the high-frequency noise. They also require a repetitive trigger which is synchronous both with the HF signal and the slow modulation. When the signal is periodic in carrier and modulation separately, it is possible to synchronously sample the signal, downconverting and compressing the carrier in combination with the modulation into an intermediate frequency (IF) data acquisition which is adapted to the modulation bandwidth. Due to the compression action of the sampling converter, also all the noise of the complete spectrum is being compressed into the IF data acquisition which reduces the signal to noise ratio.
Vector signal analyzers can demodulate a modulated high frequency tone using a mixer, shifting the carrier frequency to a low frequency, band-pass filtering it to limit the noise and digitizing it. High signal to noise ratios can be achieved in this way.
Suppose a modulated carrier is passed through an amplifier, which compresses the signal. As a result modulated fundamental and harmonics will be generated. The vector signal analyzer can demodulate the different high frequency tones, namely the fundamental and harmonics separately. However, it loses the phase coherence between these tones. Hence, there is a need for overcoming this drawback.
Nowadays harmonic signals can be measured inside a network analyzer where a reference signal is used containing phase coherent tones to normalize the harmonic signals. In this way harmonic signals can be measured and calibrated in a repeatable way. Reference is made to the papers “Measurement of Magnitude and Phase of Harmonics Generated in Nonlinear Microwave Two-Ports” (U. Lott, IEEE Trans. Microwave Theory and Techniques, vol. 37, no 10, October 1989, pp. 1506-1511) and “Measurements of time domain voltage/current waveforms at R.F. and microwave frequencies, based on the use of a Vector Network Analyzer, for the characterization of nonlinear devices. Application to high efficiency power amplifiers and frequency multipliers optimization” (D. Barataud et al., IEEE Trans. Instrumentation and Measurement, vol. 47, no 5, October 1998, pp. 1259-1264).